Mechanical or electromechanical spatial light modulators have been designed for a variety of applications, including image processing, display, optical computing, and printing. Optical beam processing for printing with deformable mirrors has been described by L. J. Hornbeck; see U.S. Pat. No. 4,596,992, issued Jun. 24, 1984, entitled "Linear Spatial Light Modulator and Printer". A device for optical beam modulation using cantilever mechanical beams has also been disclosed; see U.S. Pat. No. 4,492,435, issued Jan. 8, 1985, to M. E. Banton entitled "Multiple Array Full Width Electro-mechanical Modulator," and U.S. Pat. No. 5,661,593, issued Aug. 26, 1997 to C. D. Engle, entitled "Linear Electrostatic Modulator." Other applications of electromechanical gratings include wavelength division multiplexing and spectrometers; see U.S. Pat. No. 5,757,536, issued May 26, 1998 to A. J. Ricco et al., entitled "Electrically Programmable Diffraction Grating,".
Electro-mechanical gratings are well known in patent literature; see U.S. Pat. No. 4,011,009, issued Mar. 8, 1977 to W. L. Lama et al., entitled "Reflection Diffraction Grating Having a Controllable Blaze Angle," and U.S. Pat. No. 5,115,344, issued May 19, 1992 to J. E. Jaskie, entitled "Tunable Diffraction Grating". More recently, Bloom et al. described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV); see U.S. Pat. No. 5,311,360, issued May 10, 1994, entitled "Method and Apparatus for Modulating a Light Beam". This device was later described by Bloom et al. with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate section between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces. See U.S. Pat. No. 5,459,610, issued Oct. 17, 1995, entitled "Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate", and U.S. Pat. No. 5,808,797, issued Sep. 15, 1998, entitled "Method and Apparatus for Modulating a Light Beam". Bloom et al. also presented a method for fabricating the device; see U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, entitled "Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate".
In all embodiments of the mechanical grating device or the Grating Light Valve device (GLV) in the aforementioned patent literature, a single metallic, reflective coating with a bare upper surface has been added to the top surface of the ribbons to apply the electrostatic force required for actuation, and also to increase the efficiency of diffraction of the device by increasing the reflectivity. In high-power applications, the reflective coating also results in longer lifetime of the GLV device. A high reflectivity is important to reduce damage of the top surface of the ribbons and avoid mechanical effects that might be attributed to a significant increase in the temperature of the device due to light absorption.
A method for fabricating another embodiment of the GLV device was presented by Bornstein et al.; see U.S. Pat. No. 5,661,592, issued Aug. 26, 1997, entitled "Method of Making and an Apparatus for a Flat Diffraction Grating Light Valve". In this embodiment, a thin (&lt;500 .ANG.) dielectric layer is formed over the reflecting metallic layer on the top surface of the ribbons.
Dielectric multilayer coatings have been used on static metallic diffraction gratings to enhance diffraction efficiency and reduce absorption in the metal. D. Maystre et al. presented studies of two-layer and four-layer coatings on aluminum gratings in "Gratings for tunable lasers: using multi-dielectric coatings to improve their efficiency," published in Applied Optics, vol. 19 (1980).
The prior art does not mention the problem of the effects of the optical coating on the mechanical properties of the ribbon elements.